Winfield Hill wrote:
Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from
a decent varactor.


Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Last time I used this was with an MV104 common-cathode dual hyperabrupt,
to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature
hybrid in the usual way, coming in the 0 degree port, coming out the 180
degree port, and hanging matched reactances on the 90 degree ports.

Each section had its own inductors, and the cathodes were bypassed
heavily (1000 pF) to ground so that the two sides didn't interact too
much. The component values were 45 nH in series and 43 nH in parallel.
It was linear to within +-4 degrees, and the one section gave phase
shifts from 12 to 164 degrees, both dramatically better than I could get
with a bare varactor.

The idea is to have the varactor resonate with the series inductor just
off the low-voltage end of the range, and have the series combination
resonate with the parallel L just off the high-voltage end of the range.
Since the series-resonance doesn't even notice the parallel L, the
design equations decouple nicely, too. You adjust the placement of the
resonances to get the range and linearity desired.

Cheers,

Phil Hobbs

Winfield Hill wrote:

Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.


An inductor in series with the varactors, then another one in
parallel with the series combo can get you a very wide range of
impedance from
a decent varactor.



Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Last time I used this was with an MV104 common-cathode dual hyperabrupt,
to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature
hybrid in the usual way, coming in the 0 degree port, coming out the 180
degree port, and hanging matched reactances on the 90 degree ports.

Each section had its own inductors, and the cathodes were bypassed
heavily (1000 pF) to ground so that the two sides didn't interact too
much. The component values were 45 nH in series and 43 nH in parallel.
It was linear to within +-4 degrees, and the one section gave phase
shifts from 12 to 164 degrees, both dramatically better than I could get
with a bare varactor.

The idea is to have the varactor resonate with the series inductor just
off the low-voltage end of the range, and have the series combination
resonate with the parallel L just off the high-voltage end of the range.
Since the series-resonance doesn't even notice the parallel L, the
design equations decouple nicely, too. You adjust the placement of the
resonances to get the range and linearity desired.

Cheers,

Phil Hobbs

Winfield Hill wrote:
Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from
a decent varactor.


Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Last time I used this was with an MV104 common-cathode dual hyperabrupt,
to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature
hybrid in the usual way, coming in the 0 degree port, coming out the 180
degree port, and hanging matched reactances on the 90 degree ports.

Each section had its own inductors, and the cathodes were bypassed
heavily (1000 pF) to ground so that the two sides didn't interact too
much. The component values were 45 nH in series and 43 nH in parallel.
It was linear to within +-4 degrees, and the one section gave phase
shifts from 12 to 164 degrees, both dramatically better than I could get
with a bare varactor.

The idea is to have the varactor resonate with the series inductor just
off the low-voltage end of the range, and have the series combination
resonate with the parallel L just off the high-voltage end of the range.
Since the series-resonance doesn't even notice the parallel L, the
design equations decouple nicely, too. You adjust the placement of the
resonances to get the range and linearity desired.

Cheers,

Phil Hobbs

Winfield Hill wrote:

Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.


An inductor in series with the varactors, then another one in
parallel with the series combo can get you a very wide range of
impedance from
a decent varactor.



Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Last time I used this was with an MV104 common-cathode dual hyperabrupt,
to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature
hybrid in the usual way, coming in the 0 degree port, coming out the 180
degree port, and hanging matched reactances on the 90 degree ports.

Each section had its own inductors, and the cathodes were bypassed
heavily (1000 pF) to ground so that the two sides didn't interact too
much. The component values were 45 nH in series and 43 nH in parallel.
It was linear to within +-4 degrees, and the one section gave phase
shifts from 12 to 164 degrees, both dramatically better than I could get
with a bare varactor.

The idea is to have the varactor resonate with the series inductor just
off the low-voltage end of the range, and have the series combination
resonate with the parallel L just off the high-voltage end of the range.
Since the series-resonance doesn't even notice the parallel L, the
design equations decouple nicely, too. You adjust the placement of the
resonances to get the range and linearity desired.

Cheers,

Phil Hobbs

In article , Winfield Hill
writes
Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from
a decent varactor.

Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Thats the way its done to pull crystals a long way.
The thought/real experiment to assist is to assume that the acoustic
resonator is resistive (zero phase) at series resonance.
The maintaining circuit can then be replaced by an equivalent resistor.
The circuit with resistor should oscillate at approx the resonator
frequency.The inductor across the varicap is selected to almost tune
out/parallel resonate with the varicap.
The series inductor is phase retard to ensure the maintaining circuit
tis zero phase . Adjustment of the varicap then moves the circuit above
and below the series resonance of the resonator.




--
ddwyer

In article , Winfield Hill
writes
Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from
a decent varactor.

Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Thats the way its done to pull crystals a long way.
The thought/real experiment to assist is to assume that the acoustic
resonator is resistive (zero phase) at series resonance.
The maintaining circuit can then be replaced by an equivalent resistor.
The circuit with resistor should oscillate at approx the resonator
frequency.The inductor across the varicap is selected to almost tune
out/parallel resonate with the varicap.
The series inductor is phase retard to ensure the maintaining circuit
tis zero phase . Adjustment of the varicap then moves the circuit above
and below the series resonance of the resonator.




--
ddwyer

On Wed, 10 Dec 2003 14:39:22 +0000, Phil Hobbs
wrote:

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an LC
oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from a
decent varactor.

Thanks, is this the kind of thing you mean?



+-------+
| |
| |
| |
C| |
L1 C| |
C| |
| |
| |
V |
D1 - |
| C|
Applied DC control voltage | C| L2
Line --------------------+ C|
| |
| |
D2 - |
^ |
| |
| |
| |
+-------+

View in FP font.

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from
a decent varactor.

Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an LC
oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from a
decent varactor.


Cheers,

Phil Hobbs

that would the more total capacitance but not any larger dC/dV.....

you need even better tricks :-)

Marco

"Paul Burridge" wrote in message
...

Hi,

I'm currently working on this VCXO that achieves frequency shift by
applying DC bias to two varactor diodes connected cathode to cathode
(bias applied to the junction between them). If I can't get enough
shift with the available bias voltage, is there any problem with just
putting another pair of the same diodes in parallel with the existing
ones?
This is a ceramic resonator oscillator, BTW, so will stand a lot more
'pulling' than a xtal would, so don't worry about that aspect of it.

p.
--

"I expect history will be kind to me, since I intend to write it."
-
Winston Churchill